CA1165377A - Vacuum-tight assembly - Google Patents
Vacuum-tight assemblyInfo
- Publication number
- CA1165377A CA1165377A CA000390435A CA390435A CA1165377A CA 1165377 A CA1165377 A CA 1165377A CA 000390435 A CA000390435 A CA 000390435A CA 390435 A CA390435 A CA 390435A CA 1165377 A CA1165377 A CA 1165377A
- Authority
- CA
- Canada
- Prior art keywords
- closure member
- vacuum
- ceramic body
- expansion
- tight assembly
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J61/00—Gas-discharge or vapour-discharge lamps
- H01J61/02—Details
- H01J61/36—Seals between parts of vessels; Seals for leading-in conductors; Leading-in conductors
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J61/00—Gas-discharge or vapour-discharge lamps
- H01J61/02—Details
- H01J61/30—Vessels; Containers
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T403/00—Joints and connections
- Y10T403/21—Utilizing thermal characteristic, e.g., expansion or contraction, etc.
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/13—Hollow or container type article [e.g., tube, vase, etc.]
- Y10T428/131—Glass, ceramic, or sintered, fused, fired, or calcined metal oxide or metal carbide containing [e.g., porcelain, brick, cement, etc.]
Landscapes
- Vessels And Coating Films For Discharge Lamps (AREA)
Abstract
23,127 ABSTRACT OF THE DISCLOSURE
A vacuum-tight assembly, such as a discharge tube for a sodium vapor arc lamp, includes a high density poly-crystalline ceramic body, such as alumina or yttria, having a cavity, at least one closure member, and a seal-ing material. The closure member is formed from a molyb-denum-titanium alloy containing a small amount of nickel, cobalt, or copper. The nickel, cobalt, or copper forms a relatively low melting eutectic with titanium, thereby facilitating fabrication of closure members by sintering.
The closure member and the sealing material have thermal coefficients of expansion closely matched to the thermal coefficient of expansion of the ceramic body over a wide temperature range.
A vacuum-tight assembly, such as a discharge tube for a sodium vapor arc lamp, includes a high density poly-crystalline ceramic body, such as alumina or yttria, having a cavity, at least one closure member, and a seal-ing material. The closure member is formed from a molyb-denum-titanium alloy containing a small amount of nickel, cobalt, or copper. The nickel, cobalt, or copper forms a relatively low melting eutectic with titanium, thereby facilitating fabrication of closure members by sintering.
The closure member and the sealing material have thermal coefficients of expansion closely matched to the thermal coefficient of expansion of the ceramic body over a wide temperature range.
Description
;53'~'~
23, 127 -l-VACUUM-TIGHT SSEMBLY
Buhrer, "Vacuum-Tight ~ssembly", assignee's Canadian Patent Application 390,4~6 6, filed concurrently with the present application and assigned to the same assignee as the present application, discloses portions of the subject matter herein disclosed.
This invention relates to sealing of cavities in high density polycrystalline ceramic bodies and, more particularly, to the sealing of high pressure discharge lamps composed of alumina, yttria and the like.
Electrical discharge devices, such as high pressure sodium vapor arc lamps, commonly utilize transparent or translucent high temperature refractory tubes composed of alumina. Within the alumina tube an electric arc extends between two tungsten electrodes to which current is conducted by a hermetically sealed feedthrough assembly.
Because alumina and niobium metal have nearly equal thermal coefficients of expansion, a niobium tube or a niobium wire is used in high pressure sodium vapor lamps to conduct electrical current through the ends of the alumina arc tube. The joint between the niobium metal and the alumina is typically filled with a meltable frit based on calcium aluminate. Thus, the feedthrough assembly not only seals the discharge tube but also conducts electrical current through the end of the alumina arc tube.
While niobium is generally satisfactory as a closure member for alumina arc tubes, it is a relatively expensive metal and is in potentially short supply under certain world conditions. It is, therefore, desi~able to provide a substitute for niobium in the sealing of high pressure arc discharge tubes.
6t~
23,127 -2-~ s disclosed in copending application assignee's Canadian Patent Application 390,446-6, closure members for polycrystalline ceramic bodies can be formed from molybdenum alloys containing titanium. A preferred method o-E fabricating closure members from molybdenum alloys is by sintering. However, because of the high melting points of the molybdenum alloy and its constituents, sintering is difficult. It is, therefore, desirable to provide molybdenum-titanium alloy compositions which can be easily sintered.
Accordingly, the present invention provides a vacuum-tight assembly comprising a high density polycrystalline ceramic body having a cavity and means for sealing said cavity from the atmosphere, said ceramic body having a thermal coefficient of expansion between about 55 x 10 7/oC and 90 x 10 7/oC, said means for sealing comprising at least one sintered closure member formed from a molybdenum alloy containing between 2 and 70 atom percent titanium and between 0.1 and 5 weight percent of a metal selected from the group consisting of nickel, cobalt, copper and mixtures thereof and a sealing material, said closure member and said sealing material having thermal coefficients of expansion closely matched to the thermal coefficients of expansion of said ceramic body over a wide temperature range.
More specifically we provide an alumina discharge tube sealed by a closure member formed from a molybdenum alloy containing between 35 and 65 atom percent titanium and between 0.5 and 2 weight percent nickel.
One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which~
FIG. l is a graphic diagram illustrating the thermal expansion of alumina, yttria, and a molybdenum-titanium nickel alloy as a function of temperature; and ~3~
1,127 -3~
~ IG~ 2 is a cross-sectional view of a preferred embodiment of a vacuum-tight assembly according to the present invention.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.
, A polycrystalline ceramic body, such as a high pres-sure discharge tube, having a cavity is sealed with a molybdenum alloy and a sealing material to form a vacuum-tight assembly. Polycrystalline alu~ina, having an : 15 average thermal expansion coefficient of 81 x 10 7/oC
between the temperatures of 25C and 800C, is commonly used for discharge tubes in high pressure sodium vapor arc lamps. Yttria, having an average thermal expansion ~ coefficient of 78 x 10 /C between 25C and 800C, is ~ 20 also used in the fabrication of discharge tubes.
: The operational temperature of the seal region of high pressure sodium discharge tubes is typically between ambient temperature, or about 25C, when the device is turned off and 800C when fully warmed up. ~o avoid cracking or other destruction of the he.rmetic seal bett~7een the ceramic body and the closure member, it is necessary that the closure member and the sealing material have thermal coefficients of expansion closely matched to the thermal coefficient of expansion of the ceramic body over the operating temperature range of the seal region.
~ While high pressure sodium discharge tubes have a typical ope~ra-ting temperature range between 25C and ~00C, other vacuum tight assemblies according to the present invention can experi.ence greater or lesser operating temperature ranges and thus require matching of thermal expansion coefficients over a correspondingly greater or lesser ~ 7~~
7 _~_ temperature range. Tne c]osure members and the sealing material should have thermal coefflcients of expansion which are matched within seven percent to the thermal co-efficient of expansion of the ceramic body to provide a reliable seal.
Although the maximum temperature of the seal region of the discharge tube duxing normal operation is about 300C~ the process used to seal the discharge tube employs temperatures of about 1400C. Therefore, the closure mem--ber material must have a relatively high melting point.In addition, the material used to seal the discharge tube should have a low vapor pressure in order to avoid darken-ing of the lamp outer jacket and should be un~-eactive toward the discharge tube fill material.
'5 When molybdenum is alloyed with titanium, a suitable closure member for a cavity in a polycrystalline ceramic bocly is formed. Titanium forms a continuous series of body centered cubic solid solutions with molybdenum ahove 882C or when the titanium concentration is below a crit-ical concentration that decreases with decreasing tempera-ture, as shown by Hansen in "Constitution of Binary Alloys", McGraw-Hill, N.Y., 1958, pp 975-978. A second hexagonal phase can separate at higher titanium concentrations. In the preferred composition range for sealing alumina, the titanium concentration is between 35 and 65 atom percent and the temperature at which a second phase of ~-Ti could precipitate is between room temperature and ~00C.
Although these alloys are allowed to cool below this tem-perature range, r.o evidence of ~ny such ~-Ti phase separ-ation has been seen in x-ray diffrac-tion patterns, prob-ably because of the slow kinetics of such a low tempera-ture phase precipitation.
Molybdenum, a refractor~7 metal r has an a-~7erage thermal expansion coeificient of 55 x 10 7/oC between 25C
and 800C. Titanium has an average thermal expansion coefficient of 104 x 10 /C between 25C and 300C. By properly selecting the ratio of the component metals in `3,1~7 -5-the molyhdenum alloy, the average thermal expansion co~
efficient between 25C and 800C is adjusted upward from that of molybdenum, such that it closely matches the thermal expansion coefficient of the ceramic body to be sealed. For example, a molybdenum-titanium alloy contain-ing 50 atom percent of each element has an average ther-mal expansion coefficient of 81 x 10 7/oC between 25C
and 800C. Therefore, this alloy has a coefficient of thermal expansion substantially equal to that of alumina and can be used as a closure member for alumina arc dis-charge tubes. Other thermal coefficients of expansion between 55 x lC 7/oC and 90 x 10 7/oC can be matched by varying the concentration of titanium relative to molybde-num. The thermal coefficient of expansion of the mol~b-denum alloy increases more or less linearly from ; 55 x 10 7~oc as the concentration of titanium is increase~
A molybdenum alloy containing between 2 and 70 atom percent titanium can be used as the closure member for sealing a cavity in a high density polycrystalline ceramic bocly wherl the ceramic body has a thermal coefficient of expansion between about 55 x 10 7/oC and 90 x 10 7/oC.
When the ceramic body is alumina or yttria, i-t is pre-ferred that the molybdenum alloy contain between 35 and 65 atom percent titanium. When the titanium concentration is outside the range of 35 to 65 atom percent, the resul-tant molybdenum alloy does not have thermal characteris~
tics which sufficiently match those of alumina or yttria to pro~Jide reliable sealing.
One preferred method of fabricating molybdenum-; 30 titanium alloy closure members is by slntering. Ho~ever, because oE the high melting point of the molybdenum alloy and its constituents, sintering is difficult. A desirable sintering temperature is about 1500C. It has been found that the addition to the molybdenum-titanium alloy of a small amoun-t of nickel, cobalt or copper facllitates sintering of the molybdenum alloy by forming a liquid in-teryranular phase at d sintering temperature of 1500C.
~i.5;3~
~`,12.7 -6-The sinterincJ aids of the present invention, nickel, cobalt, copper an~. mixtures thereof, form with titarAium a eutectic which melts at about 1000C. The sintering aids are used in concentra-tions of between 0.1 and 5 weight percent= At sinteriny aid concentrations above :: 5 percent, the composition deforms or melts completely during sintering. A preferred sintering aid concentra-tion is between about 0.5 weight percent and 2 weight percent. One particularly preferred sintering aid is nickel.
In fabricating molybdenum alloy closure members by sintering, the alloy component metal powders and the sin-te.ring aid powders are ground together and pressed into a large pellet for a first heating cycle. The pressure used is about 90,000 lbs. per square inch and the firing cycle is 7 minutes at 1500C in vacuum. The large pellet is reground wnen cooled and the powder pressed in a small hardened steel die having provision for forming holes to accommodate electrode rods as described hereinafter.
Electrode rods are then inserted into the pressed part : and the assembly is sintered for 7 minutes at 1500C.
Compositions with 2 weight percent nickel, 32.5 weight percent titanium and 65,5 weight percent molybde-num have been used to fabricate sintered parts having a : 25 thermal expansion coefficient of 86 x 10 /C. This ther-mal expansion coefficient is slightly higher than that of the molybdenum alloy without ni.ckel and is due to the presence of a solidified eutectic grain boundary phase containing Ti2Ni. While the parts contai.ning 2 weight percent nickel exhibit porosities of less than 1 percent, the sintered parts are somewhat brittle. Compositions with 1 weight percent nickel, 33 weight percent titanium and 66 weight percent molybdenum form a homogenous solu-tion after sintering with no grain boundary phase. The porosity of sintered p~rts with 1 weight percent nickel is less than 10 pexcent.
~,127 7 Referriny now to Fig. 1, there is shown a graphic diagram illustrating the expansion curves of al.umina, ~ yttria and a molybdenum-titanium alloy containing 66 ~ weight percent molybdenum, 33 weiy'nt percent titani~n and S 1 weiyhc percent nickel as a function of temperature. The closely matched thermal characteristics of alumina and the molybdenum-titanium alloy are illustrated in Fig~ 1. Fig.
1 also illustrates the matching in thermal characteristics ; betwePn yttria and the molybdenum-titanium alloy.
The construction of a vacuum-tight feedthrough assem-bly for a high pressure sodium vapor lamp is shown iIi Fig. 2. A discharge tube 40, formed from alumina, yttria or other transparent ceramic material, includes a caJity 42 which contains the larnp fil' material and an opening through an end thereof. A closure member 44 for~ned from a sintered molybdenum-titanium alloy as described herein-above is located in the opening in the discharge tube 40.
The closure member 44 has a generally cylindrical portion 46 which is slightly smaller than the opening in the dis-charge tube 40 and a lip portion 48 which is larger than the opening i~ the discharge tube 40. Tne lip portion 48 holds the closure member 44 in position during the seal-ing process. An electrode assembly includes a tungsten rod 50 and a tungsten coil 52 impregnated with ~missive activator material such as calcium barium tungstater The tungsten rod 50 and a molybdenum connection lead 54 are pressed into holes on opposite sides of the closure mem-ber 44 and are bonded therein during sintering as describ-ed hereinabove or welded in place after sintering.
During sealing of the discharge tube 40, a sealing material 56 is placed between the closuxe memb~r 44 and the discharge tube 40. The sealing material 56 is typic-ally a meltable frit based on calcium aluminate. The assembly is then heated to about 1400~C to melt the seal-ing material 56 and cause it to flow into the space be-tween the discharge tube 40 and the closure member 44, thereby providing a ~acuum-tight feedthrough assen!bly.
.~
~,127 -8-; The followlng examples are for the purpose of fur~
ther il~ustrating and explaining the present invention and are not to be taken as limiting in any regard. Unless otherwise indicated, al] parts and percentages are by weight~
Example I
~ molybdenum alloy was prepared from 65.5 percent - Sylvania type 390/325 mesh molybdenum, 32.5 percent ~MI
Company type ~lI-TI 020/100 mesh titanium and 2 percent -325 mesh nickel powder. The powders were mixed, pressed into a 1/2 inch diameter pellet and sintered at 1500C
for 5 to 10 minutes~ The pellet was reground, pressed in a 3/16 inch diameter die at 92,000 psi and sintered a second time at 1500C for 5 to 10 minutes. The pieoes held s'nape well and were 99.5 percent of the theoretical density of the nickel-free molybdenum alloy. X-ray dif-fraction studies of the sintered pieces showed a major phase of molybdenum and titanium in solid solution and minor phases of Ti2Ni and Ti20. Metallographic and scan-ning electron microprobe studies showed the molybdenum-titanium alioy grains to contain some nickel but much of the nickel was concentrated in the grain boundary phase.
Example II
The double sintering process described in Example I
was repeated with a nickel-free composition wherein molyb denum and titanium were in a 2 to 1 weight ratio. The density of the sintered pieces was only 72 percent of the theoretical density of the nickel free molybdenum-alloy.
Fxample III
___ A molybdenum alloy was prepared from 66 percent Sylvania type 390/325 mesh molybdenum, 33 percent RMI
Company type RMI-TI-020/100 mesh titanium and 1 percent -325 mesh nickel powder. The powders were mixed, pressed and sintered at 1500C for 5 to 10 minutes. The sintered pieces were then reground, pressed in a 3/16 inch diameter die at 92,000 ysi and resintered at 1500C for 5 to 10 minutes. The resultant sintered pieces containing 1 `3,1~7 _9_ percent nickel were 92.3 percent of the theoretical den-sity of the nickel~free molybdenum alloy. X-ray diffrac-tion studies of the pieces showed a single phase solid solution of molybdenum and titanium. Metallographic studies showed no grain boundary phase indicating that the nickel remains in solid solution with the molybdenum and titanium.
Four cylindrical specimens of this alloy having diameters of 0.17 inches and a total lenyth of 0.796 inches were measured using a dila~ometer calibrated against molybdenum. The thermal expansion of the molyb-denum-titanium-nickel alloy is plotted in Fig. 1. The average thermal coefficient of expansion hetween 25C and 800UC was determined to be 84.1 x 10 7/oC.
xample IV
A molybdenum alloy was prepared from 64 percent Sylvania type 390/325 mesh molybdenum, 32 percent RMI
Company type RMI-TI-020/100 mesh titanium and 4 percent -325 mesh nickel powder. The powders were mixed, pressed intb a 1/2 inch diame-ter pellet and sintered at 1500~C
for 5 minutes. The pieces were then reground, pressed into a 3/16 inch diameter die at 92,000 psi and resintered at 1500C for 5 minutes. The sintered pieces were 95.5 percent of the theoretical density of the nickel free molybdenum alloy. However, the parts showed some defor-mation during sintering.
Example V
In this example, a high pressure sodium discharge lamp was constructed with a sintered molybdenum alloy closure member. A closure member containing 65.5 percent molybdenum, 32.5 percent titanium and 2 percent nickel was prepared in accordance with the procedure of Example I.
A sintered piece having the general configuration of the closure member shown in Fiyure 3 was ground to fit a 0.125 inch hole in a 150 watt polycrystalline alumina arc tube. A standard electrode and a connection lead were "
attached to the closure member. A preformed frit rlng o~
calcium alurninace was placed be-tween the arc tube and the c~osure member. The arc tube was heated to l~Q0C in the re~ion of the closure member in an inert gas filled fur-nace to allow the frit ring to melt and form the sealbetween the dlscharge tube and the closure member. The seal was found to be hermetic under helium leak testing.
The discharge tube was then filled with 30 mg of a sodium amalgam and 20 torr of argon and the opposite end of the discharge tube was sealed with a standard niobium feedthrough using standard sealing methods. The discharge tube was tested and found to be operational.
The discharge tube was then temperature cycled to test the integrity of the seal between the alumina arc tube and the molybdenum alloy. A temperature cycle con-; sisted of 5 minutes on followed by 5 minutes off. ~fter 13,450 cycles, the seals were still intact and the dis-charge -tube was still operational without degradation o light output or starting behavior.
~0 Example VI
Closure members containing 66 percent molybdenum, 33 percent titanium and 1 percent nickel were prepared in accordance with the procedure of Example ~II. A high pressure sodium discharge lamp was constructed as describ-ed in Example V except that both ends of the arc tubewere sealed with molybdenum-titanium-nickel closure mem-~ bers. The seals were hermetic and the arc tube wa~ fully ; operational.
While there has been shown and described what is at present consldered the preferred embodiments o-f the in-; vention, it will be obvious to those skilled in the art that various changes and modifications may be made there-in without departing from the scope of the invention as defined by the appended claims.
23, 127 -l-VACUUM-TIGHT SSEMBLY
Buhrer, "Vacuum-Tight ~ssembly", assignee's Canadian Patent Application 390,4~6 6, filed concurrently with the present application and assigned to the same assignee as the present application, discloses portions of the subject matter herein disclosed.
This invention relates to sealing of cavities in high density polycrystalline ceramic bodies and, more particularly, to the sealing of high pressure discharge lamps composed of alumina, yttria and the like.
Electrical discharge devices, such as high pressure sodium vapor arc lamps, commonly utilize transparent or translucent high temperature refractory tubes composed of alumina. Within the alumina tube an electric arc extends between two tungsten electrodes to which current is conducted by a hermetically sealed feedthrough assembly.
Because alumina and niobium metal have nearly equal thermal coefficients of expansion, a niobium tube or a niobium wire is used in high pressure sodium vapor lamps to conduct electrical current through the ends of the alumina arc tube. The joint between the niobium metal and the alumina is typically filled with a meltable frit based on calcium aluminate. Thus, the feedthrough assembly not only seals the discharge tube but also conducts electrical current through the end of the alumina arc tube.
While niobium is generally satisfactory as a closure member for alumina arc tubes, it is a relatively expensive metal and is in potentially short supply under certain world conditions. It is, therefore, desi~able to provide a substitute for niobium in the sealing of high pressure arc discharge tubes.
6t~
23,127 -2-~ s disclosed in copending application assignee's Canadian Patent Application 390,446-6, closure members for polycrystalline ceramic bodies can be formed from molybdenum alloys containing titanium. A preferred method o-E fabricating closure members from molybdenum alloys is by sintering. However, because of the high melting points of the molybdenum alloy and its constituents, sintering is difficult. It is, therefore, desirable to provide molybdenum-titanium alloy compositions which can be easily sintered.
Accordingly, the present invention provides a vacuum-tight assembly comprising a high density polycrystalline ceramic body having a cavity and means for sealing said cavity from the atmosphere, said ceramic body having a thermal coefficient of expansion between about 55 x 10 7/oC and 90 x 10 7/oC, said means for sealing comprising at least one sintered closure member formed from a molybdenum alloy containing between 2 and 70 atom percent titanium and between 0.1 and 5 weight percent of a metal selected from the group consisting of nickel, cobalt, copper and mixtures thereof and a sealing material, said closure member and said sealing material having thermal coefficients of expansion closely matched to the thermal coefficients of expansion of said ceramic body over a wide temperature range.
More specifically we provide an alumina discharge tube sealed by a closure member formed from a molybdenum alloy containing between 35 and 65 atom percent titanium and between 0.5 and 2 weight percent nickel.
One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which~
FIG. l is a graphic diagram illustrating the thermal expansion of alumina, yttria, and a molybdenum-titanium nickel alloy as a function of temperature; and ~3~
1,127 -3~
~ IG~ 2 is a cross-sectional view of a preferred embodiment of a vacuum-tight assembly according to the present invention.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.
, A polycrystalline ceramic body, such as a high pres-sure discharge tube, having a cavity is sealed with a molybdenum alloy and a sealing material to form a vacuum-tight assembly. Polycrystalline alu~ina, having an : 15 average thermal expansion coefficient of 81 x 10 7/oC
between the temperatures of 25C and 800C, is commonly used for discharge tubes in high pressure sodium vapor arc lamps. Yttria, having an average thermal expansion ~ coefficient of 78 x 10 /C between 25C and 800C, is ~ 20 also used in the fabrication of discharge tubes.
: The operational temperature of the seal region of high pressure sodium discharge tubes is typically between ambient temperature, or about 25C, when the device is turned off and 800C when fully warmed up. ~o avoid cracking or other destruction of the he.rmetic seal bett~7een the ceramic body and the closure member, it is necessary that the closure member and the sealing material have thermal coefficients of expansion closely matched to the thermal coefficient of expansion of the ceramic body over the operating temperature range of the seal region.
~ While high pressure sodium discharge tubes have a typical ope~ra-ting temperature range between 25C and ~00C, other vacuum tight assemblies according to the present invention can experi.ence greater or lesser operating temperature ranges and thus require matching of thermal expansion coefficients over a correspondingly greater or lesser ~ 7~~
7 _~_ temperature range. Tne c]osure members and the sealing material should have thermal coefflcients of expansion which are matched within seven percent to the thermal co-efficient of expansion of the ceramic body to provide a reliable seal.
Although the maximum temperature of the seal region of the discharge tube duxing normal operation is about 300C~ the process used to seal the discharge tube employs temperatures of about 1400C. Therefore, the closure mem--ber material must have a relatively high melting point.In addition, the material used to seal the discharge tube should have a low vapor pressure in order to avoid darken-ing of the lamp outer jacket and should be un~-eactive toward the discharge tube fill material.
'5 When molybdenum is alloyed with titanium, a suitable closure member for a cavity in a polycrystalline ceramic bocly is formed. Titanium forms a continuous series of body centered cubic solid solutions with molybdenum ahove 882C or when the titanium concentration is below a crit-ical concentration that decreases with decreasing tempera-ture, as shown by Hansen in "Constitution of Binary Alloys", McGraw-Hill, N.Y., 1958, pp 975-978. A second hexagonal phase can separate at higher titanium concentrations. In the preferred composition range for sealing alumina, the titanium concentration is between 35 and 65 atom percent and the temperature at which a second phase of ~-Ti could precipitate is between room temperature and ~00C.
Although these alloys are allowed to cool below this tem-perature range, r.o evidence of ~ny such ~-Ti phase separ-ation has been seen in x-ray diffrac-tion patterns, prob-ably because of the slow kinetics of such a low tempera-ture phase precipitation.
Molybdenum, a refractor~7 metal r has an a-~7erage thermal expansion coeificient of 55 x 10 7/oC between 25C
and 800C. Titanium has an average thermal expansion coefficient of 104 x 10 /C between 25C and 300C. By properly selecting the ratio of the component metals in `3,1~7 -5-the molyhdenum alloy, the average thermal expansion co~
efficient between 25C and 800C is adjusted upward from that of molybdenum, such that it closely matches the thermal expansion coefficient of the ceramic body to be sealed. For example, a molybdenum-titanium alloy contain-ing 50 atom percent of each element has an average ther-mal expansion coefficient of 81 x 10 7/oC between 25C
and 800C. Therefore, this alloy has a coefficient of thermal expansion substantially equal to that of alumina and can be used as a closure member for alumina arc dis-charge tubes. Other thermal coefficients of expansion between 55 x lC 7/oC and 90 x 10 7/oC can be matched by varying the concentration of titanium relative to molybde-num. The thermal coefficient of expansion of the mol~b-denum alloy increases more or less linearly from ; 55 x 10 7~oc as the concentration of titanium is increase~
A molybdenum alloy containing between 2 and 70 atom percent titanium can be used as the closure member for sealing a cavity in a high density polycrystalline ceramic bocly wherl the ceramic body has a thermal coefficient of expansion between about 55 x 10 7/oC and 90 x 10 7/oC.
When the ceramic body is alumina or yttria, i-t is pre-ferred that the molybdenum alloy contain between 35 and 65 atom percent titanium. When the titanium concentration is outside the range of 35 to 65 atom percent, the resul-tant molybdenum alloy does not have thermal characteris~
tics which sufficiently match those of alumina or yttria to pro~Jide reliable sealing.
One preferred method of fabricating molybdenum-; 30 titanium alloy closure members is by slntering. Ho~ever, because oE the high melting point of the molybdenum alloy and its constituents, sintering is difficult. A desirable sintering temperature is about 1500C. It has been found that the addition to the molybdenum-titanium alloy of a small amoun-t of nickel, cobalt or copper facllitates sintering of the molybdenum alloy by forming a liquid in-teryranular phase at d sintering temperature of 1500C.
~i.5;3~
~`,12.7 -6-The sinterincJ aids of the present invention, nickel, cobalt, copper an~. mixtures thereof, form with titarAium a eutectic which melts at about 1000C. The sintering aids are used in concentra-tions of between 0.1 and 5 weight percent= At sinteriny aid concentrations above :: 5 percent, the composition deforms or melts completely during sintering. A preferred sintering aid concentra-tion is between about 0.5 weight percent and 2 weight percent. One particularly preferred sintering aid is nickel.
In fabricating molybdenum alloy closure members by sintering, the alloy component metal powders and the sin-te.ring aid powders are ground together and pressed into a large pellet for a first heating cycle. The pressure used is about 90,000 lbs. per square inch and the firing cycle is 7 minutes at 1500C in vacuum. The large pellet is reground wnen cooled and the powder pressed in a small hardened steel die having provision for forming holes to accommodate electrode rods as described hereinafter.
Electrode rods are then inserted into the pressed part : and the assembly is sintered for 7 minutes at 1500C.
Compositions with 2 weight percent nickel, 32.5 weight percent titanium and 65,5 weight percent molybde-num have been used to fabricate sintered parts having a : 25 thermal expansion coefficient of 86 x 10 /C. This ther-mal expansion coefficient is slightly higher than that of the molybdenum alloy without ni.ckel and is due to the presence of a solidified eutectic grain boundary phase containing Ti2Ni. While the parts contai.ning 2 weight percent nickel exhibit porosities of less than 1 percent, the sintered parts are somewhat brittle. Compositions with 1 weight percent nickel, 33 weight percent titanium and 66 weight percent molybdenum form a homogenous solu-tion after sintering with no grain boundary phase. The porosity of sintered p~rts with 1 weight percent nickel is less than 10 pexcent.
~,127 7 Referriny now to Fig. 1, there is shown a graphic diagram illustrating the expansion curves of al.umina, ~ yttria and a molybdenum-titanium alloy containing 66 ~ weight percent molybdenum, 33 weiy'nt percent titani~n and S 1 weiyhc percent nickel as a function of temperature. The closely matched thermal characteristics of alumina and the molybdenum-titanium alloy are illustrated in Fig~ 1. Fig.
1 also illustrates the matching in thermal characteristics ; betwePn yttria and the molybdenum-titanium alloy.
The construction of a vacuum-tight feedthrough assem-bly for a high pressure sodium vapor lamp is shown iIi Fig. 2. A discharge tube 40, formed from alumina, yttria or other transparent ceramic material, includes a caJity 42 which contains the larnp fil' material and an opening through an end thereof. A closure member 44 for~ned from a sintered molybdenum-titanium alloy as described herein-above is located in the opening in the discharge tube 40.
The closure member 44 has a generally cylindrical portion 46 which is slightly smaller than the opening in the dis-charge tube 40 and a lip portion 48 which is larger than the opening i~ the discharge tube 40. Tne lip portion 48 holds the closure member 44 in position during the seal-ing process. An electrode assembly includes a tungsten rod 50 and a tungsten coil 52 impregnated with ~missive activator material such as calcium barium tungstater The tungsten rod 50 and a molybdenum connection lead 54 are pressed into holes on opposite sides of the closure mem-ber 44 and are bonded therein during sintering as describ-ed hereinabove or welded in place after sintering.
During sealing of the discharge tube 40, a sealing material 56 is placed between the closuxe memb~r 44 and the discharge tube 40. The sealing material 56 is typic-ally a meltable frit based on calcium aluminate. The assembly is then heated to about 1400~C to melt the seal-ing material 56 and cause it to flow into the space be-tween the discharge tube 40 and the closure member 44, thereby providing a ~acuum-tight feedthrough assen!bly.
.~
~,127 -8-; The followlng examples are for the purpose of fur~
ther il~ustrating and explaining the present invention and are not to be taken as limiting in any regard. Unless otherwise indicated, al] parts and percentages are by weight~
Example I
~ molybdenum alloy was prepared from 65.5 percent - Sylvania type 390/325 mesh molybdenum, 32.5 percent ~MI
Company type ~lI-TI 020/100 mesh titanium and 2 percent -325 mesh nickel powder. The powders were mixed, pressed into a 1/2 inch diameter pellet and sintered at 1500C
for 5 to 10 minutes~ The pellet was reground, pressed in a 3/16 inch diameter die at 92,000 psi and sintered a second time at 1500C for 5 to 10 minutes. The pieoes held s'nape well and were 99.5 percent of the theoretical density of the nickel-free molybdenum alloy. X-ray dif-fraction studies of the sintered pieces showed a major phase of molybdenum and titanium in solid solution and minor phases of Ti2Ni and Ti20. Metallographic and scan-ning electron microprobe studies showed the molybdenum-titanium alioy grains to contain some nickel but much of the nickel was concentrated in the grain boundary phase.
Example II
The double sintering process described in Example I
was repeated with a nickel-free composition wherein molyb denum and titanium were in a 2 to 1 weight ratio. The density of the sintered pieces was only 72 percent of the theoretical density of the nickel free molybdenum-alloy.
Fxample III
___ A molybdenum alloy was prepared from 66 percent Sylvania type 390/325 mesh molybdenum, 33 percent RMI
Company type RMI-TI-020/100 mesh titanium and 1 percent -325 mesh nickel powder. The powders were mixed, pressed and sintered at 1500C for 5 to 10 minutes. The sintered pieces were then reground, pressed in a 3/16 inch diameter die at 92,000 ysi and resintered at 1500C for 5 to 10 minutes. The resultant sintered pieces containing 1 `3,1~7 _9_ percent nickel were 92.3 percent of the theoretical den-sity of the nickel~free molybdenum alloy. X-ray diffrac-tion studies of the pieces showed a single phase solid solution of molybdenum and titanium. Metallographic studies showed no grain boundary phase indicating that the nickel remains in solid solution with the molybdenum and titanium.
Four cylindrical specimens of this alloy having diameters of 0.17 inches and a total lenyth of 0.796 inches were measured using a dila~ometer calibrated against molybdenum. The thermal expansion of the molyb-denum-titanium-nickel alloy is plotted in Fig. 1. The average thermal coefficient of expansion hetween 25C and 800UC was determined to be 84.1 x 10 7/oC.
xample IV
A molybdenum alloy was prepared from 64 percent Sylvania type 390/325 mesh molybdenum, 32 percent RMI
Company type RMI-TI-020/100 mesh titanium and 4 percent -325 mesh nickel powder. The powders were mixed, pressed intb a 1/2 inch diame-ter pellet and sintered at 1500~C
for 5 minutes. The pieces were then reground, pressed into a 3/16 inch diameter die at 92,000 psi and resintered at 1500C for 5 minutes. The sintered pieces were 95.5 percent of the theoretical density of the nickel free molybdenum alloy. However, the parts showed some defor-mation during sintering.
Example V
In this example, a high pressure sodium discharge lamp was constructed with a sintered molybdenum alloy closure member. A closure member containing 65.5 percent molybdenum, 32.5 percent titanium and 2 percent nickel was prepared in accordance with the procedure of Example I.
A sintered piece having the general configuration of the closure member shown in Fiyure 3 was ground to fit a 0.125 inch hole in a 150 watt polycrystalline alumina arc tube. A standard electrode and a connection lead were "
attached to the closure member. A preformed frit rlng o~
calcium alurninace was placed be-tween the arc tube and the c~osure member. The arc tube was heated to l~Q0C in the re~ion of the closure member in an inert gas filled fur-nace to allow the frit ring to melt and form the sealbetween the dlscharge tube and the closure member. The seal was found to be hermetic under helium leak testing.
The discharge tube was then filled with 30 mg of a sodium amalgam and 20 torr of argon and the opposite end of the discharge tube was sealed with a standard niobium feedthrough using standard sealing methods. The discharge tube was tested and found to be operational.
The discharge tube was then temperature cycled to test the integrity of the seal between the alumina arc tube and the molybdenum alloy. A temperature cycle con-; sisted of 5 minutes on followed by 5 minutes off. ~fter 13,450 cycles, the seals were still intact and the dis-charge -tube was still operational without degradation o light output or starting behavior.
~0 Example VI
Closure members containing 66 percent molybdenum, 33 percent titanium and 1 percent nickel were prepared in accordance with the procedure of Example ~II. A high pressure sodium discharge lamp was constructed as describ-ed in Example V except that both ends of the arc tubewere sealed with molybdenum-titanium-nickel closure mem-~ bers. The seals were hermetic and the arc tube wa~ fully ; operational.
While there has been shown and described what is at present consldered the preferred embodiments o-f the in-; vention, it will be obvious to those skilled in the art that various changes and modifications may be made there-in without departing from the scope of the invention as defined by the appended claims.
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A vacuum-tight assembly comprising:
a high density polycrystalline ceramic body having a cavity; and means for sealing said cavity from the atmosphere;
said ceramic body having a thermal coefficient of expansion between about 55x10-7/°C. and 90x10-7/°C;
said means for sealing comprising a sintered closure member formed from a molybdenum alloy containing between 2 and 70 atom percent titanium and between 0.1 and 5 weight percent of a metal selected from the group consisting of nickel, cobalt, copper and mixtures thereof; and a sealing material interposed between said ceramic body and said closure member for providing a seal therebetween, said closure member and said sealing material having thermal coefficients of expansion of said ceramic body over a wide temperature range.
a high density polycrystalline ceramic body having a cavity; and means for sealing said cavity from the atmosphere;
said ceramic body having a thermal coefficient of expansion between about 55x10-7/°C. and 90x10-7/°C;
said means for sealing comprising a sintered closure member formed from a molybdenum alloy containing between 2 and 70 atom percent titanium and between 0.1 and 5 weight percent of a metal selected from the group consisting of nickel, cobalt, copper and mixtures thereof; and a sealing material interposed between said ceramic body and said closure member for providing a seal therebetween, said closure member and said sealing material having thermal coefficients of expansion of said ceramic body over a wide temperature range.
2. A vacuum-tight assembly as defined in claim 1 wherein said ceramic body includes a material selected from the group consisting of alumina and yttria.
3. A vacuum-tight assembly as defined in claim 2 wherein said closure member contains between 0.5 and 2 weight percent of a metal selected from the group consisting of nickel, cobalt, copper and mixtures thereof.
4. A vacuum-tight assembly as defined in claim 3 wherein said closure member contains between 35 and 65 atom percent titanium.
5. A vacuum-tight assembly as defined in claim 4 wherein said ceramic body comprises a cylindrical dis-charge tube and wherein said closure member is adapted for sealing an end of said discharge tube.
6. A vacuum-tight assembly as defined in claim 2 wherein said closure member contains between 0.5 and 2 weight percent nickel.
7. A vacuum-tight assembly as defined in claim 1 wherein said closure member and said sealing material have thermal coefficients of expansion closely matched to the thermal coefficient of expansion of said ceramic body over the operating temperature range of said assembly.
8. A vacuum-tight assembly as defined in claim 3 wherein said closure member and said sealing material have thermal coefficients of expansion matched within seven percent to the thermal coefficient of expansion of said ceramic body over the temperature range 25°C. to 800°C.
9. A vacuum-tight assembly as defined in claim 1 wherein said sealing material is a meltable frit.
10. A vacuum-tight assembly as defined in claim 9 wherein said meltable frit is calcium aluminate.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US209,162 | 1980-11-21 | ||
| US06/209,162 US4334628A (en) | 1980-11-21 | 1980-11-21 | Vacuum-tight assembly |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1165377A true CA1165377A (en) | 1984-04-10 |
Family
ID=22777618
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000390435A Expired CA1165377A (en) | 1980-11-21 | 1981-11-19 | Vacuum-tight assembly |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4334628A (en) |
| EP (1) | EP0052843B1 (en) |
| JP (1) | JPS57111944A (en) |
| CA (1) | CA1165377A (en) |
| DE (1) | DE3172628D1 (en) |
Families Citing this family (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4587144A (en) * | 1982-05-13 | 1986-05-06 | Hillel Kellerman | Low loss, compression, hermetic glass-to-metal-seal and method |
| US4537323A (en) * | 1984-01-09 | 1985-08-27 | Gte Laboratories Incorporated | Mo-Ti members with non-metallic sintering aids |
| US4542843A (en) * | 1984-04-27 | 1985-09-24 | Gte Laboratories Incorporated | Method of friction welding a lamp feedthrough assembly |
| US4584454A (en) * | 1984-04-27 | 1986-04-22 | Gte Laboratories Incorporated | Method of welding a lamp feedthrough assembly; and apparatus therefor |
| US4704557A (en) * | 1986-03-11 | 1987-11-03 | The United States Of America As Represented By The United States Department Of Energy | Cermet insert high voltage holdoff for ceramic/metal vacuum devices |
| US6154188A (en) * | 1997-04-30 | 2000-11-28 | Candescent Technologies Corporation | Integrated metallization for displays |
| JP3528649B2 (en) * | 1998-03-09 | 2004-05-17 | ウシオ電機株式会社 | Lamp cermets and ceramic discharge lamps |
| US6853119B2 (en) * | 2001-08-02 | 2005-02-08 | Osram Sylvania Inc. | Double layer electrode coil for a HID lamp and method of making the electrode coil |
| JP4943646B2 (en) * | 2004-11-19 | 2012-05-30 | 大倉工業株式会社 | Adhesive sheet for moisture-permeable floor curing |
| AT501721B1 (en) * | 2005-03-11 | 2006-11-15 | Konstantin Technologies Ges M | EVAPORATOR SOURCE FOR EVAPORATING ALKALI / ERDALKALIMETALLEN |
| US7923932B2 (en) * | 2007-08-27 | 2011-04-12 | Osram Sylvania Inc. | Short metal vapor ceramic lamp |
| US7710038B2 (en) * | 2007-12-21 | 2010-05-04 | Osram Sylvania Inc. | Ceramic discharge vessel having molybdenum alloy feedthrough |
Family Cites Families (12)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3061664A (en) * | 1959-11-13 | 1962-10-30 | Kimble Glass Co | Glass-to-metal seals and method of fabricating same |
| US3275358A (en) * | 1963-03-14 | 1966-09-27 | Gen Electric | Glass-to-metal and glass-to-ceramic seals |
| US4004173A (en) * | 1965-12-27 | 1977-01-18 | Sydney Alfred Richard Rigden | Niobium alumina sealing and product produced thereby |
| US3435180A (en) * | 1967-08-03 | 1969-03-25 | Gen Electric | Method of making a molybdenum-tungsten thimble seal |
| US3588573A (en) * | 1967-12-29 | 1971-06-28 | Westinghouse Electric Corp | Alumina-rare earth oxide ceramic to metal seals for containing high temperature vapors |
| US3588577A (en) * | 1969-03-17 | 1971-06-28 | Gen Electric | Calcia alumina magnesia baria seal composition |
| FR2102865A5 (en) * | 1970-08-27 | 1972-04-07 | Eclairage Lab | |
| US3848151A (en) * | 1973-10-23 | 1974-11-12 | Gen Electric | Ceramic envelope lamp having metal foil inleads |
| GB1465212A (en) * | 1975-05-12 | 1977-02-23 | Gen Electric | Electric discharge lamps |
| NL174683C (en) * | 1975-09-11 | 1984-07-16 | Philips Nv | HIGH PRESSURE GAS DISCHARGE LAMP. |
| HU174714B (en) * | 1977-01-06 | 1980-03-28 | Egyesuelt Izzolampa | Electric discharge tube |
| US4197957A (en) * | 1978-12-26 | 1980-04-15 | Gte Laboratories Incorporated | Vacuum tight assembly |
-
1980
- 1980-11-21 US US06/209,162 patent/US4334628A/en not_active Expired - Lifetime
-
1981
- 1981-11-13 EP EP81109680A patent/EP0052843B1/en not_active Expired
- 1981-11-13 DE DE8181109680T patent/DE3172628D1/en not_active Expired
- 1981-11-19 CA CA000390435A patent/CA1165377A/en not_active Expired
- 1981-11-20 JP JP56185542A patent/JPS57111944A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0052843A1 (en) | 1982-06-02 |
| EP0052843B1 (en) | 1985-10-09 |
| DE3172628D1 (en) | 1985-11-14 |
| US4334628A (en) | 1982-06-15 |
| JPS57111944A (en) | 1982-07-12 |
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